I am using Photoshop CS6 Extended on Lenovo Thinkpad running Win 8. I found out the following problem:
1. Open a high res image as smart object.
2. Scale it down to 50 % in PS
3. The image looks a bit pixelated.

Now if the smart object is rasterised first. Then scaled down. The resulting image looks very smooth.

Apparently, scaling down a rasterised layer is better looking than scaling down a smart object layer.